Abstract. N2O abundances ranging from 0.5 to 6 ppbv were observed in the polar
upper stratosphere/lower mesosphere by the MIPAS instrument on the Envisat
satellite during the Arctic and Antarctic winters in the period July 2002 to
March 2004. A detailed study of the observed N2O-CH4 correlations shows
that such enhancements cannot be explained by dynamics without invoking an
upper atmospheric chemical source of N2O. The N2O enhancements observed
at 58 km occurred in the presence of NOx intrusions from the upper
atmosphere which were related to energetic particle precipitation. Further,
the inter-annual variability of mesospheric N2O correlates well with
observed precipitating electron fluxes. The analysis of possible chemical
production mechanisms shows that the major part of the observed N2O
enhancements is most likely generated under dark conditions by the reaction
of NO2 with atomic nitrogen at altitudes around 70–75 km in the presence
of energetic particle precipitation (EPP). A possible additional source of
N2O in the middle and upper polar atmosphere is the reaction of
N2(A3Σu+), generated by precipitating electrons, with O2,
which would lead to N2O production peaking at altitudes around
90–100 km. N2O produced by the latter mechanism could then descend to
the mesosphere and upper stratosphere during polar winter. The estimated
fraction of EPP-generated N2O to the total stratospheric N2O inside the
polar vortex above 20 km (30 km) never exceeds 1% (10%) during the
2002–2004 winters. Compared to the global amount of stratospheric N2O,
the EPP-generated contribution is negligible.